I think Relational Erosion describes when someone was a great friend of yours years ago but when you meet them again face-to-face, they don't feel as warm toward you as they used to. Mostly they've moved on, found more exciting friends, heard bad things about you, and just anything that with the passage of time, makes them less of a friend than they used to be.
It's like how a car rusts and the gasoline gets gummed up when it's in a long storage. When you try to start it again, it doesn't sound as well as the last time you did.
Same for a coastline- you notice the yard on your oceanfront home is about 5 feet shorter than before thanks to coastal erosion. (Scary thought; those houses aren't worth a loaf of bread if they'll be eroded away any year now. That's what I heard of houses on Britain's coastlines.)